U.S. Pat. No. 4,036,162, which is incorporated herein by reference, discloses a marine propulsion device which comprises an engine having an exhaust port for discharging exhaust gas, and which also comprises a lower unit having an exhaust tube in communication with the exhaust port. The lower unit includes a cavitation plate submerged in water during idle engine operation, and an exhaust outlet in communication with the exhaust tube. The exhaust outlet affords discharge of the exhaust gas below the cavitation plate. The lower unit includes an outer wall including an outlet, which outer wall outlet is located above the cavitation plate and submerged in water during idle engine operation. The lower unit also includes a passage in communication with the exhaust tube and the outer wall outlet. The passage affords, during reverse engine operation, discharge of a portion of the exhaust gas from the exhaust tube out the outer wall outlet.
U.S. Pat. No. 4,668,199, which is incorporated herein by reference, discloses an exhaust system for an outboard motor, which includes a main exhaust passageway extending through a partially water filled chamber in the driveshaft housing. An inlet idle relief passage connects the top of the chamber with the main exhaust passageway and an outlet passage connects the top of the chamber with the atmosphere. The system thus defines an effective exhaust silencer for the idle exhaust.
U.S. Pat. No. 5,954,554, which is incorporated herein by reference, discloses an outboard drive that involves an improved exhaust system that increases the reverse thrust produced by the outboard drive. The exhaust system includes a first exhaust passage and a second exhaust passage that stems from a first exhaust passage. A flow control device operates within the exhaust system to control exhaust gas flow through second passage depending upon the drive condition (either forward or reverse) of the outboard drive. The flow control device permits exhaust gas flow through the second passage when the outboard drive operates in reverse, while inhibiting exhaust gas flow through the second passage when the outboard drive operates under a forward drive condition. In this manner, the improved exhaust system reduces exhaust gas back pressure and thrust degradation due to exhaust gas entrainment in the propeller when the outboard drive operates in reverse.
U.S. Pat. No. 7,195,528, which is incorporated herein by reference, discloses in an outboard motor exhaust system having a first exhaust gas passage discharging engine exhaust gas into water and a shift actuator operating a shift mechanism to establish one from among a forward position, a reverse position and a neutral position, a second exhaust gas passage is branched from the first exhaust gas passage at a location above the water and an exhaust valve installed in the second exhaust gas passage and connected to the shift mechanism to be opened when the reverse position is established. The exhaust valve is alternatively opened by an exhaust valve actuator installed separately from the shift actuator. With this, it becomes possible to prevent the decrease in thrust produced during reverse boat travel by the engine exhaust gas being sucked in by a propeller, without degrading shift feel.